Electronic timer



NOV- 1, 1960 D. E. GRAVES ELECTRONIC TIMER Filed Jan. 13, 195s United States Patent ELECTRONIC TIMER Donald E. (iraves, Wauwatosa, Wis., assignor to General Electric Company, a corporation of New York Filed Jan. 13, 1958, Ser. No. 708,442

8 Claims. (Cl. Z50-95) The present invention relates in general to interval timing, and has more particular reference to electronic timing apparatus especially well suited to control the supply of actuating energy to an electrical load during selectively variable time intervals, the apparatus being particularly well suited for use in controlling the operation of X-ray generating tubes during accurately timed operating intervals.

X-ray generating tubes function to produce X-rays in response to the activation of the anode of the tube, as an X-ray source, by the impngement thereon of electrons emitted by and from the cathode of the mbe. By apply ing an electrical potential between the cathode and the anode of .the tube, cathode emitted electrons maybe caused to travel thence toward and to impinge upon the anode at high velocity, thereby constituting the anode as an X-ray source. Operating potential may be applied between the anode and cathode by connecting the same with a suitable electrical power source, through a ysupply circuit embodying control means, such as a relay switch or contactor. X-ray tubes, accordingly, may be operated for X-ray production during precisely timed operating periods, by accurately determining the interval during which operating power is applied between the anode and cathode of the tube, through the control switch.

Precision interval timing means, of course, may be employed to actuate the tube controlling relay switch at precisely determined star-ting and stopping instants; and it will be obvious that the accuracy of timed tube operation will be dependent upon the responsespeed and other operating characteristics of the `relay switch that may be selected for use in controlling the operation of the tube.

An important object of the present invention is to provide for the operation r of an electrical load controlling relay switch in novel fashion resulting in speedy, reliable and substantiallyuniform response action of the switch.

Another important object of the'invention is to provide an interval timer comprising a relay switch having an actuating coil interconnected in the common anodecathode line of a push-pull relay controlling circuit; a further object being to provide a pair of thyratron valves in the push-pull circuit; a still further object being to provide a normally open circuit conditioning relay switch in the push-pull circuit in combination with circuitry operable to close said switch to condition the circuitl for operation, after a predetermined time delay interval following closure of an associated control switch, and a normally open thyratron starting relay switch controlled to close after closure of the circuit conditioning switch to thereby complete a biasing circuit to apply a thyratron firing bias on the control grids of the thyratrons;ryet another object of the invention being to provide a normally closed switch in the biasing circuit vand timing means for opening the same to disable the thyratrons upon the expiration of a timed interval of selected duration following closure of the star-ting switch; another object being to vprovide auxiliary safety timing means ,operable to `open a normally closed relay switch connected in the push-pull circuit to rce thereby disable the thyratrons upon the expiration of the timed interval.

Another object is to provide for disabling at least portions of the timing system when the load device is operated at reduced voltage, as when an X-ray tube is used in fluoroscopy.

Another object is to provide for connecting adjustable resistance units in the push-pull circuit in order to control the pull in and drop out times of the controlled relay switch, thereby allowing the same to be adjusted to make and break the controlled power circuit at desired instants in the controlled power wave.

Another important object of the invention is to mainvtain the thyratron valves of the switch actuating circuit normally in cut-off or in active condition by application of a negative bias upon the screen grids of said valves, while connecting the plates thereof to the opposite ends of the secondary winding of a power supply transformer; a further object ybeing to provide for applyinga positive potential to the control grids of the thyratrons whenever operation of the timing system is initiated, so as to instantly iire whichever of the thyratrons has a positive plate condition at such instant, thereby energizing the actuating coil of the relay switch means, in order to promptly start the controlled X-ray tube in operation.

Referring to the drawings:

The single gure comprising the drawings is a schematic wiring diagram of an electronic timer embodying the present invention.

To illustrate the invention, the drawings show an electronic timer system 11, especially well suited for controlling the operation of an X-ray generating tube 12, although it will be obvious that the invention is not necessarily limited to the specific nature of the controlled apparatus. As shown, electrical power for the operation of the electronic timer 11, as well as the controlled device 12, may be supplied from a suitable power source 13 through power supply conductors L-1. and L-2, under the control of a master switch S-1, power for the operation of the controlled load device 12 being delivered through a supply circuit, connected between the conductors L-1 and LeZ, said supply circuit containing the switch contactors MS of a control relay having a switch operating coil MC operable when energized to close the switch contactors MS, in order to supply power to the load device 12. Power may be delivered to the load device through a transformer T-1 and a rectifying network R-l, although, if desired, the load device 12 may be connected directly in the power supply circuit in series with the switch contactors MS.

In order to control the relay switch MS to open and close at the beginning and end of a precisely delimited load energizing interval, the timing system 11 may comprise a relay operating circuit 14, and timing circuitry 15 embodying a time delay section 16, a signal starting section 17, a timing section 18- and a safety timing section 19, said sections respectively comprising thyratron tubes V-l, V-2, V-3y and V-4, and associated circuit components.

The relay operating circuit 14 comprises a pair of thyratron tubes V-S and V-6 energized in push-pull relation by power supplied from the conductors L-1 and L-2, through a transformer T-2. To this end, the anodes of the thyratron tubes V-S and V-6 are connected respectively to the opposite ends of a secondary winding 20 of the transformer T-2, said secondary winding having a center tap electrically connected with the interconnected cathodes of the thyratron tubes V-5 and V-6 through a series circuit comprising a normally closed relay switch S-6a, an adjustable resistance 21, a normally closed relay switch S-4a, a normally open relay switch S-Sb, and the switch actuating coil MC.

'Ihe control grids of the thyratron valves V-S and V-6 are connected through condensers 22 with the cathodes of the valves, which in turn may be connected with the negative potential side of a uni-directional power source, such as a rectilier network R-Z, the control grids of the thyratron tubes being also connected with the positive potential side of the power source R-Z through resistors 23, a resistor 24 being connected across the positive and negative potential sides of the power source, to form a load resistor and voltage dividing network.

The screen grids o-f the thyratron tubes V-S and V-6 are electrically interconnected with the negative potential side of a uni-directional power source, such as the rectifier network R-3, the positive side of which is connected with the cathodes of the thyraton tubes, a filter condenser being interconnected across the positive and negative potential sides ofthe source R-S.

The rectifier network R-Z is connected in an energizing-circuit extending between the conductors L-1 and L-Z, said energizing circuit including a normally closed relay switch S-3, a normally open relay switch S-Za and a Vnormally open relay switch S-5a. The rectilier network R-3 may be energized from a secondary winding 25 of the transformer T-Z, the network being interconnected between a center tap and one end of said winding. A warning buzzer Z6 may be electrically connected in lseries with a relay switch S4b between the opposite ends ofthe winding 25.

The time delay section 16 comprises a rel-ay switch having an operating coil C-1 and associated switches including a normally closed switch S-la, and normally open switches S-1b, S1c and S-ld. The relay coil C-l is connected in series between the anode of the thyratron tube V-1 and the power supply conductor L-1, the cathode of the thyratron tube being electrically connected with the power supply conductor `L-Z through a normalvly open relay switch S-7 and a manually operable switch 27 for controlling the operation of an anode driving motor M, which is employed Where the X-ray tube forming the load device 12 embodies a rotary type anode requiring the introduction of a time delay interval during vwhich the anode is` brought to operating speed prior to the application of actuating power to the load device. This time delay interval also permits the cathodes of the X-ray tube and of the thyratron tubes to be properly excited prior to the start of a timed exposure interval.

Where required, however, the anode driving motor M may be connected in a circuit extending from the conductor L-Z through the manually operable switch 27, the normally open relay switch S-7, the motor M and -a control switch network 28, comprising the normally closed relay switch S-la and the normally open relay switch S-1b connected in series with a resistor 29, the switch S-1a being connected in parallel across the resistor 29 and the switch S-lb.

The normally open switch S-1c is connected in parallel with a resistor 30 and between the power conductors L-1 and L2 in series with the primary winding 31 of a transformer having a secondary winding connected to cleliver energizing power to the cathode of the X-ray tube .forming theV load device 12. Accordingly, when the system is in operation, with power supplied in the con ductors L-1 and L-Z, the transformer winding 31 will normally be energized, through the resistor Sil, at a level to operate the cathode of the X-ray tube at less than X-ray generating intensity, in order to condition the tube for operation.

The normally open relay switch S-ld is connected between the power supply conductors L-l and lf2, in series with the operating coil C-VS of a relay embodying the normally open switches S-Sa and S5b, and a manually operable exposure switch 32.

The time delay section 16 also includes a time delay `relay having an operating coil C-7 electrically interconnected directly between the power supply Conductors 4 L-l and L-Z, said relay embodying the normally open switch S7.

The control grid of the thyratron tube V-1 is electrically connected with the power supply conductor L-Z through a timing circuit 33 comprising a resistor 34 and a condenser 35 connected in parallel relation with the resistor, said control grid being also connected through a condenser 36 with the cathode and screen grid of the valve and with the power supply conductor L-l through a resistor 37.

The signal starting section 17 comprises relay switch means embodying the normally open switch S-Za, another normally open switch S-Zb, a normally closed switch S-Zc, and a switch actuating coil C-Z which is electrically connected in series between the power supply conductor L-1 and the anode of the thyratron tube V-2, the cathode of which is electrically connected with the power supply conductor L-2 through the normally open switch S-Sa. The control grid of the thyratron tube V-Z is electrically connected with the power supply conductor L-Z through a timing circuit 33' comprising a resistor 38 and a condenser 39 connected in parallel relation with the resistor, saidcontrol grid being also electrically connected through a condenser 40 with the cathode Aand screen grid of the thyratron tube and with the power supply conductor L-1 through a resistor 37.

The timing section 18 comprises relay switch means embodying the normally closed relay switch S-3 and an associated actuating coil C-3 which is electrically interconnected between the power supply conductor L-l and the anode of the thyratron tube V-S', the cathode of which is electrically connected with the power supply conductor L-Z. The cathode of the thyratron tube V-3 is also connected withY the screen grid of said tube and with the power supply conductor L-1 through a condenser 41, a rectier 42 and a ballast resistance 43, the resistance elements of -a pair of potentiometers 44 and 45 being interconnected in parallel relation with respect to the condenser 41. The control grid of the thyratron tube V-3 is connected with the power supply conductor L-Z through a resistor and a timing condenser 46, an

ladjustable timing resistance 47 being connected in series nected through the normally closed switch S-2c with the ,adjustable elements of the potentiometer 44.

The safety timing section 19 comprises relay switch means embodying the normally closed switch S-4a, the normally open switch S-4b and an associated actuating coil C-4 which is connected between the power supply conductor L-1 and the anode of the thyratron tube V-4 in series with a rectilier 48 and a ball-ast resistor 49. The cathode of the thyratron tube V-4 is connected with the power supply conductor L-2, with the screen grid of the tube and with the power Supply conductor L-1 through a condenser 50, the normally open switch S-Sc, the rectiiier 48 and the ballast resistor 49, a resistor 51 being connected in parallel with the condenser 50. The control grid of the thyratron tube V-4 may be connected with the cathode of the tube through a condenser 52. The control grid of the thyratron is also connected with the power supply conductor L -Z through a resistor and a timing condenser 53, an adjustable timing resistance 54 being connected in series with a normally open relay Yswitch S6c `across the condenser 53. The side of the condenser 53 which -is connected with the' control grid of the thyratron tube V-4 is connected through a normally closed relay switch S-6b with the adjustable element of the potentiometer 45.

`circuit supplying power to the load device 12, so that the coil C-6 may be supplied with energizing power Whenever and so long las power is supplied to the load device 12, providing the control switch S-Sb remains in closed position. The relay means containing the normally closed switch S-8b may also embody a normally open switch S-Sa connected across the relay switch MS in series with a voltage dropping resistor 55, said switches S8a and S-8b being operatively associated with an actuating coil C-8 adapted to be energized from a suitable source of power 56 under the control v'of -a normally open switch 57, which may convenientlyv comprise a foot switch adapted to be closed to operate the X-ray generating tube at relatively low voltage as a ray source for the fluoroscopic examination of patients.

The timing apparatus of the present invention may be placed in operation by first closing the master control switch S-l, to thereby apply electrical energy on the power supply conductors L-l and L-2. Upon closure of the switch S-1, the transformer T-2 will become energized to supply operating power to the relay actuating circuit 14 including the thyratron valves V-5 and V-6 in push-pull relation. The actuating coil C-7 of the time delay relay will `also be energized to cause closure of the switch S-7 after a `'delay of the order of thirty Vseconds to lpermit the entire system to become conditioned for operation. After closure of the switch S-7, the manually operable anode motor control switch 27 may be closed `in order to start the motor M in operation through the circuit controlled by the normally closed relay switch S-la.

During the thirty second time delay interval following closure of switch S-l and prior to the closure of switch S-7, the cathode of the thyratron valve V-1 remains disconnected from the power supply conductor L-2 so that the cathode and anode of the thyratron tube are at approximately the same potential and the tube can not fire. The control grid of the thyratron, however, is connected to the conductor L-2 through the timing network 33 so that the control grid of the thyratron becomes positive with respect to the cathode during every other half cycle of the power wave supplied through the switch -S-1. The grid-cathode combination, under such co-nditions, functions as a diode rectiiier, thereby charging the timing condenser 35 at a negative potential of the order of ninety volts. As a consequence, when switch 27 is closed, following closure of the switch S-7, the grid `and cathode of the thyratron tube change polarity together, thereby breaking the charging path to the con- -denser 35 and allowing the same to discharge through 'the resistor 34. Where the resistor 34 has a value of the order of 1.5 megohms, the condenser 35 may discharge Iduring an interval of the order of 0.6` seconds, thereby permitting the thyratron valve V-l to tire yand to energize the relay coil C-l. As a consequence, the normally closed switch S-la will be caused to open and the normally open switches S-lb, S1c, and S-ld will be caused to close approximately six-tenths of a second after closure of the switch 27, such interval being of Isufiicient duration to permit the motor M to reach operating speed. Closure of the switch S-lb in series with the resistor 29 simultaneously with the opening of the switch S-1a, will reduce the voltage at which energy is applied on the motor M from -a relatively high starting voltage to a relatively lower operating voltage.

Closure of the switch S-lc will cause the winding 31 of the cathode exciting transformer to be energized at an operating voltage relatively higher than that at which the transformer winding is initially energized through .the resistor 30 following closure of the switch S-1 and prior to the firing of the thyratron tube V-1.

Closure of the normally open switch S-ld in response vto the tiring of the thyratron V-l will place the relay actuating coil C5 in condition to be energized in response to manual closure of the exposure switch 32. Accordingly, it will kbe seen that the timing apparatus 6 may not be operated to initiate delivery of power to the load device 12 until the expiration of a time delay interval of duration sufficient to condition the system for operation, including the initial warming of the electron emitting cathode of the load device and the bringing of the `anode motor to operating speed.

Closure of the exposure switch 32, after the thyratron tube V-l has tired, energizes the relay coil C-S to close the normally open relay switches S-Sa, S-Sb, and S5c. Closure vof the switch S-5a connects the cathode of the thyratron tube V-,2 with the power supply conductor L-Z, thereby causing the thyratron valve to re and energize t-he relay operating coil C-2. So ylong as the switch S-Sa remains open, the thyratron tube V-2 can not fire but its grid-cathode combination'serves as a diode rectifier to charge the condenser 39 to a negative potential of the order of ninety volts. Closure of the switch S-Sa breaks the charging path to the condenser 39 and permits it to discharge through the resistor 38, thus permitting the thyratron-tube V-Z to tire after a time delay interval which depends upon the value of resistance 38 in the timing circuit 33, said interval being preferably of the order of one-thirtieth of a second.

Activation of the relay coil C-2 upon the tiring of the thyratron tube V-2 causes closure of the normally open switch S-Za and S-2b and the opening of the normally closed switch S-2c. Closure of the switch S-Za energizes the rectifying network R-Z by connecting it between the power supply conductors L-l and L-2 through the switches S-3, S-Za and S-Sa, thereby applying uni-directional potential of positive character through the control resistors 23 and upon the control grids of the thyratron tubes V-S and V-6. The condensers 22, 36, 40 and 52 serve to balance the anodecathode capacitance of the tubes V-1, V-2, V-4, Ve and V-6, and thus prevent erratic operation thereof.

Upon closure of the main switch S-1, it will be seen that the transformer T-Z, through its secondary winding 25, will have put the rectifying network R-S in operation to apply a relatively negative potential upon the screen grids of the thyratron valves V-S and V-6, the relatively positive side of the rectifying network being electrically connected with the cathodes of the thyratron tubes. As a consequence, the thyratron tubes are normally held inactive and prevented from firing. The application on the control grids of the thyratron valves of a relatively positive potential from the network R-Z will cause the thyratrons V-S and V-6 to re as each plate goes electrically positive and supply pulsating electrical energy to the coil MC, the normally open switch S-Sb having previously been closed in response to the successive closure of the relay switch S-ld and the manually operable exposure switch 32.

The function of the resistance 24 is to provide a load resistor and voltage divider on the rectifier network R-2, and in turn to furnish the positive bias upon the control grids of the thyratron valves. This bias voltage is transferred from the resistor 24 through the grid resistors 23. The Variable resistor 21 serves to determine the amount 0f current which flows initially in the switch actuating coil MC, thereby controlling the speed at which the associated switch MS closes. The resistor 21 thus can be adjusted to determine the position in the energy wave at which the switch MS is caused to close.

Prior to the opening of the relay switch S-2c, a negative bias voltage of the order of ten volts will have been applied upon the condenser 46 by operation of the rectitier 42 through the potentiometer 44. When the thyratron V-2 operates to cause closure of the switch S-2b and simultaneously to open the switch S-2c, the condenser 46 is caused to discharge through the adjustable resistor 47, thereby causing the thyratron tube V-3 to re after a time relay interval of duration determined by the adjusted resistance value of the resistor 47. The firing of the thyratron tube V-3 energizes the relay coil -7 C-3, thereby opening the normally closed switch S-S which .serves to connect the rectifier networkR-Z between the power supply conductors L-1 and L-Z. The supply of relatively positive bias potential upon the control grids of the thyratrons'V-S and V-6 is thus terminated and the thyratrons, consequently, become disabled, thereby cutting olf the delivery of energizing power to the switch actuating coil MC and permitting the switch MS to open.

The relay operating circuit 14 preferably includes an adjustable resistor 58 connected across and in parallel relation with respect to the normally closed relay switch S-6a, such resistor consequently becoming connected in series in the circuit of the switch actuating coil MC in response to the opening of the normally closed switch S-a. Capacitor 80 is merely for the purpose of suppressing any arc that might occur across S-6a when it opens to by-pass current through adjustable impedance 58. The normally closed relay switches S-6a and S6b are opened whenever their associated actuating coil C6 is energized by closure of the switch MS. When thus connected in series circuit with vthe switch actuating coil MC, the resistor 58 serves to control the speed at which the switch MS opens. If desired, a condenser 58' may be connected across the coil MC to control relay operating time. The resistor thus permits adjustment of the apparatus to cause the switch MS to open at a desired time instant on the voltage wave of power delivered through the switch in order to reduce arcing as the switch is opened. The switch S-Sb serves as a safety switch which is closed only by operation of the relay coil C-S following firing of the thyratron tube V-l and closure of the exposure switch 32, failing which the switch S-Sb remains open preventing the coil MC from becoming energized.

The safety timer section 19 serves to disable the relay operating circuit 14 and thereby cause the switch MS to open in the event of failure of the timing section 18. To this end, the safety timer circuit is energized by half wave rectified energy supplied by the rectifier 48 which is connected with the filter capacitor 50 through the relay switch S-5c upon closure of the exposure switch 32. Rectied power is employed in order to keep the thyratron tube V-4 in operation after it has fired. The switch S-Sc is merely employed to reduce the plate potential upon the thyratron, so that small surges during switching will not interfere with the operation of the timer. The switch also provides a simple means for extinguishing the thyratron valve even though it has rectified power applied to it. When the capacitor 50 is not connected in circuit, the rectified power supply may have a value of approximately thirty-five volts upon the anode of the thyratron, but when the switch S-Zc is closed the potential may be somewhat in excess of a hundred volts.

When the switch MS is closed in response to the activation of the relay operating circuit 14, the relay coil C-ti becomes energized, providing the switch S-Sb remains closed, thereby not only opening the switch S-6a to apply the resistor 58 in circuit with the switch actuating coil MC, but also to open the switch Sv6b in order to remove a negative bias which may be of the order of nine volts from the timing condenser y53, such bias being supplied from the rectifier 42 through the potentiometer 4S. The coil C-6, when energized, serves also to close the normally open yswitch S-6c in order to permit the condenser 53 to be discharged through the adjustable timing resistance 54, thereby allowing the thyratron V-4 to fire after a time delay interval determined by the adjusted value of the resistance 54. When the thyratron tube V-4 fires, the relay coilC-'4 becomes energized, thereby causing the normally closed switch S-4a to open, thereby deactivating the switch operating coil MC, in the event that the timing section 18 shall have failed to function vfor the opening of the switch S-S and the disablement of the rectifying network R-Z and the thyratrons V-5 andl V-6 controlled by said network. Y z

The purpose'of the safety timing section 19 thus is to provide. an auxiliary timing Afunction parallel with that furnished by the main timing section 18." Should theV main Vtiming section18k fail to operate, the safety timing seetion will not only open the operating circuit of the switch actuating coil MC at the switch S-4a, but will also close the switch S-4b forthe` operation of the warning buzzer 26. The system thus provides dual control of the pushg pull circuit 14, one'by way of the bias circuit rectiier system R4. and theother by the relay switch S-4a.

Means is provided for disabling the safety timing section while the X-ray tube forming the load device 12 is employed as a ray source for uoroscopic purposes. During fluoroscopy it is unnecessary to utilize any' part o f the timing circuit; but control of the X-ray tube during fluoroscopy is accomplished by employing the foot switch 57 which when closed serves to energize a relay coil C-8 which is operatively associated with the normally open switch S-Sa, and the normally closed switch S-Sb. Accordingly, by'closing the foot' switch 57, the coil C-8 functions to closethe open switch S-Sa, thereby energizing the load device V12, for low voltage operation by means of the resistor 55. At`the same time, the coil C-8 voperates to open the switch S-Sb, thereby disabling Vthe coil C-6 which causes the switches S-6a and S-6b to regain their normally closed position and the switch S-6c toV regain its normally open position, whereby the thyratron V-4 is held in inactive condition thereby disabling the safety timing section 19.

A principal feature of the timing apparatus is the em'- Vployment of a push-pull thyratron circuit which enables use of a conventional relay switch comprising the coil MC and the associated switch MS to control intervals as short as one-sixtieth second with precise accuracy. Heretofore it has been possible to obtain such timing accuracy only with special impulse timing apparatus. The present invention, however, provides a timer which approaches the accuracy of impulse timing equipment for intervals of exceedingly short duration, such as intervals of the order of one-sixtieth of a second. The present invention makes possible such precision by controlling the grid of half ampere thyratrons, and by initially placing approximately ten times the normal current through a standard relay switch coil MC to increase the speed at which it will operate, while immediately dropping the excessive current down to a normal value within two impulses of energizing power, to thereby prevent the operating coil MC of the conventional contactor from burning out, by action of the switch S-6a. At the same time, the present invention affords means for controlling the contactor switch to open at a particular instant in the energy cycle of the power that is controlled by the switch. The circuitry obtains control of a pulsating unidirectional current which can be controlled during each impulse as a result of the push-pull relationship of the thyratrons V-S and V-o. In addition to controlling each impulse, the current flowing in the switch MS can be controlled by means of a simple resistance circuit, instead of by phase shifting.

It is thought that the invention and its numerous attendantradvantages will be fully understood from the foregoing description, and it is obvious that numerous changesmay be made in the form, construction and arrangement ofthe several parts without departing from the spirit or scope of the invention, or sacrificing any of its attendant advantages, the form herein disclosed being a preferred embodiment for the purpose of illustrating the invention.

The invention is hereby claimed as follows:

l. A timing system for energizing a load during a precisely delimited period comprising an operating circuit including normally inactive electron ow means and the load to be energized, means operable to apply power on said system, circuit conditioning switch means operable to condition the circuit for operation after an interval of selected duration following application of power on said system, circuit activating switch means operable to activate said electron ilow means to energize said load, after the lapse of an interval of selected duration following operation of said conditioning switch means, and timing means operable to inactivate said electron ow means at the end of a selected interval following activation thereof.

2. A timing system for energizing a load during a precisely delimited period comprising an operating circuit including normally inactive electron ow means and the load to be energized, means operable to apply power on said system, circuit conditioning switch means operable to condition the circuit for operation after an interval of selected duration following application of power on said system, said electron ow means embodying a control element, circuit activating switch means operable to apply a bias upon said control element, whereby to activate said electron ow means to energize said load, after the lapse of an interval of selected duration following operation of said conditioning switch means, timing means operable to remove said bias from said control element, whereby to inactivate said electron iiow means at the end of a selected interval following activation thereof, and safety means operable to open the circuit at the conclusion of said selected interval, in the event of failure of said timing means to inactivate the electron flow means.

3. A timing system for an X-ray tube having a cathode `and a motor driven rotary anode, comprising the combination, with a relay switch having contacts for controlling the supply of power to the X-ray tube and an actuating coil, of an operating circuit embodying said coil and normally inactive electron flow means operable to supply energizing power to the coil, a biasing circuit operable to activate the flow means, master switch means for applying power on said system, an anode motor energizing circuit, relay means including a normally open switch in said motor energizing circuit to condition the same for operation in response to the application of power on said system, a control switch for activating said motor energizing circuit, time delay means operable to alter the amount of power applied upon the motor, to energize the cathode to condition same for operation, and also to condition ya timed interval start circuit for operation, after the lapse of a selected time delay interval following activation of the motor energizing circuit, an operating coil in said start circuit and a normally open switch closable to condition said biasing circuit for operation to activate said flow means, and a time delay relay circuit operable to activate said biasing circuit after expiration of an interval of desired duration following activation of said timed interval start circuit.

4. A timing system for an X-ray tube having a cathode and a motor driven rotary anode, comprising the combination, with a relay switch having contacts for controlling the supply of power to the X-ray tube and an actuating coil, of an operating circuit embodying said coil and normally inactive electron flow means operable to supply energizing power to the coil, a biasing circuit operable to activate the ow means, an anode motor energizing circuit, a control switch for activating said motor energizing circuit, time delay means operable to alter the amount of power applied upon the motor, to energize the cathode to condition the same for operation, and also to condition a timed interval start circuit for operation, after the lapse of a selected time delay interval following activation of the motor energizing circuit, an operating coil in said start circuit and a normally open switch closable to condition said biasing circuit for operation to activate said ow means, a time delay relay circuit operable to activate said biasing circuit after expiration of a time delay interval of desired duration following activation of said timed interval start circuit, and timing means operable to disable said biasing circuit at the end of a precisely delimited interval following activation of said biasing circuit.

5. A timing system for an X-raytube having a cathode and a motor driven rotary anode, comprising the combination, with a relay switch having contacts for controlling the supply of power to the X-ray tube and an actuating coil, of an operating circuit embodying said coil and normally inactive electron flow means operable to supply energizing power to the coil, a normally open conditioning switch in said operating circuit, a biasing circuit operable to activate the ow means, an anode motor energizing circuit, a control switch for `activating said motor energizing circuit, a timed interval start circuit, time delay means operable to energize the cathode to condition the same and the timed interval start circuit for operation, said time delay means serving also to close said conditioning switch after the lapse of -a selected time delay interval following activation of the motor energizing circuit, a normally open switch closable to condition said biasing circuit for operation to activate said ow means, `a time delay relay circuit operable to activate said biasing circuit after expiration of a time delay interval of desired duration following activation of said timed interval start circuit, and timing means operable to disable said biasing circuit at the end of a precisely delimited interval following activation of said biasing circuit.

6. A timing system `for energizing an electric load during a precise interval comprising a pair of thyratron tubes connected in push-pull relationship with an alternating power source to provide a series load circuit including a relay coil load to be energized, said thyratron tubes each including an anode, cathode and control grid, means normally holding said tubes in cuto condition, means operable to apply an activating bias on the control grids of both of said tubes simultaneously during a load energizing interval to render conductive whichever tube has a relatively positive anode, a first adustable impedance means in series with said load, said impedance means controlling the magnitude of current through said relay coil load and establishing the elective operating point thereof, and means for removing said activating bias to restore said thyratron tubes to cutoff condition and to terminate said interval.

7. The invention set forth in claim 6 including in said load circuit a serially connected normally closed Contact in shunt with a second adustable impedance means, said contact opening in response to energization of said relay coil to connect said first and second impedance means in series with each other for controlling current through `Said load as the interval proceeds, whereby said second impedance means controls the de-energizing time of said relay coil load.

`8. The invention set forth in claim 6 including a capacitor shunted across said relay coil load, said capacitor absorbing energy from the relay coil following its deenergization to allow elective control thereof during extremely short intervals.

References Cited in the tile of this patent UNITED STATES PATENTS 2,339,902 Askers Ian. 25, 1944 2,353,980 Weisglass July 18, 1944 2,431,705 Nemet Dec. 2, 1947 2,473,640 Faulk June 21, 1949 2,504,865 Morgan et al Apr. 18, 1950 2,785,346 Large Mar. 12, 1957 2,809,296 Godbarsen Oct. 8, 1957 

